There has been a general trend in the coating art to utilize polymer systems containing higher levels of solids content. Several benefits can be derived from such use. The enamels require less solvent addition during preparation, an energy reduction is available in the drying process, there is a potential for operating with fewer passes, and there is the possiblity for increasing spindle count. However, several problems become apparent when higher solids enamels are utilized on conventional equipment.
For example in the die applied method, the viscous materials do not permit high wire speed, these dies are expensive and the viscous material, which is constantly being recycled, entraps air readily, thereby creating an inferior wire product.
A roller applied coating apparatus is used in some installations. The wire is coated with an excess of enamel by a roller and is subsequently introduced to a series of wiper rolls. A relationship between the roller speeds and wire speed exists whereby some of the enamel is removed leaving the desired wet enamel distributed around the wire. This method requires a low viscosity enamel for best results as a highly viscous material would not flow easily from the roller to the wire. The large exposed surface area of the rollers, coupled with rollers turning several hundred RPM makes this system costly and environmentally dangerous due to large solvent evaporation. Once again, with higher solids coatings, the viscosity increases rapidly with the loss of solvent, and the recirculation of relatively large quantities of enamel can cause excessive air entrapment and contamination. Precise viscosity control is required with the roller system and the excessive evaporation makes the system sensitive and requires additional amounts of a solvent blend to be added to compensate for the losses.